1. Field of the Invention
The present invention relates to osteosynthesis devices for the spinal column, the devices comprising a plate and a mechanism for locking a bone screw or anchoring member in position.
2. Description of the Prior Art
U.S. Pat. No. 5,876,402 relates to an osteosynthesis plate comprising through-holes of conical shape capable of housing a bone screw with a completely spherical head to which is clipped, so as to form a ball-joint connection. A split coupling element of conical exterior shape complementing that of the hole is provided. A circlip reduces the aperture of the through-hole. A similar clip and groove arrangement is shown in U.S. Pat. Nos. 5.879,389 and 6,102,952.
In U.S. Pat. No. 5,876,402, the bone screw is placed in the coupling element prior to insertion in the plate. Upon insertion, the split coupling element opens up the circlip. This circlip closes up again once the coupling element has passed through. The coupling element is thus held captive in the through-hole. Final clamping of the anchoring member in position is achieved by the frictional wedging of the coupling element in the bottom of the cone.
In such a system, the number of parts makes the clamping-in position of the anchoring members weak. In addition, the clamping does not occur at the instant when the circlip closes up again after the passage of the coupling element. This leads to the risk of the assembly becoming unclamped, which is prejudicial to the patient.
FIGS. 5 and 6 of U.S. Pat. Nos. 5,879,389 and 6,102,952 show a split-ring for installation in a groove after the bone screw or anchor has been installed in the bone.
One of the objects of the present invention is to provide a spinal implant which is easier to fit while at the same time being reliable.
With a view to achieving this objective, the present invention envisages an implant, particularly for the spinal column, comprising a joining member such as a plate exhibiting openings or orifices, bone-anchoring members such as bone screws capable of being accommodated in the orifices and at least one split ring capable of holding the members in the orifices. The split ring can come into direct contact with the anchoring member or members to hold the member or members in the orifices.
Thus, the number of parts involved in the locking is reduced and this locking can be made more reliable. Advantageously, the joining member comprises a plate and the orifices comprise an opening with a spherical seat.
Preferably, each anchoring member or bone screw comprises a complementary spherical part capable of coming into contact with the spherical seat. Thus, the surgeon has, at his disposal, freedom to orient the anchoring member angularly with respect to the joining member or plate, thus allowing him to optimize the anchorage.
Advantageously, the anchoring members or bone screws comprise driving means such as a drive socket.
In one embodiment the split ring is preferably common to at least two orifices and includes a driving means, which driving means comprise openings. In another embodiment, the split ring is specific to each orifice in the plate.
In one embodiment the split ring is preferably common to at least two orifices and includes a driving means, which driving means comprise openings. In another embodiment, the split ring is specific to each orifice in the plate.
Advantageously, the split ring has a variable cross-section so as to optimize its flexibility. Thus, the ring will deform more readily when introducing the head into the orifice. The amount of time taken and the number of operations required during surgical intervention will be reduced.
The bone plate, screw and ring may be supplied as part of a screw locking system for bone plates to be used by a surgeon. The bone plate has at least one opening therein, and normally a plurality of openings, for receiving a bone screw or bone anchor. The openings extend along an axis from a top surface to a bottom bone contacting surface of the plate. Each opening has an upper region with a first diameter with a groove formed therein having a depth defined by a diameter greater than the first diameter. The plate has a lower region including a seat for the bone screw. The bone screw has a head with a maximum diameter which is smaller than the first diameter, thereby allowing the screw head to pass through that region of the opening.
An expandable ring is provided which is pre-mounted in the groove and having, when relaxed and unexpanded, an external diameter greater than the first diameter, but smaller than the groove diameter. The expandable ring has an internal diameter when relaxed and unexpanded, smaller than both the first and the head diameters. The expandable ring is capable of expanding into the groove so that the internal diameter expands to be larger than or equal to the screw head diameter while, at the same time, the external diameter is less than or equal to the groove diameter.
With this geometry, the split-ring can be pre-mounted in the groove and the screw can be inserted, shank first into the bone plate from the upper non-bone contacting surface and, upon engagement between the head of the screw and the split-ring, the split-ring expands into the groove, allowing the head to pass therethrough. Once the screw head has passed through this split-ring, it contracts under its natural spring tension. When the ring relaxes to its unexpanded state, it prevents the bone screw from backing out of the plate by the engagement of an undersurface of the split-ring and an upwardly facing surface on the bone screw.
The openings in the lower portion of the bone plate have a part spherical seat portion located between the groove and the bottom bone contacting surface of the plate with an opening in the bottom plate surface to allow the shank of the bone screw to pass through. The screw head has a corresponding part spherical surface extending from the shank of the screw towards the upwardly facing surface of the screw. Upon insertion of the screw through the plate, the screw head engages the part spherical seat on the bone plate. At that point the screw head is below the split-ring groove. The bone screw shank can be threaded in any well known fashion and may include an axial groove to enable the screw to be self-boring and self-tapping. The bone screw may include an internal bore extending along the longitudinal axis of the screw which includes threads for engaging a pull out tool should removal of the screw be necessary.
In order to enhance the locking system""s ability to prevent the screw from backing out of the bone plate, both the groove and split-ring have complementary inclined surfaces extending towards the upper surface of the bone plate upon moving towards the center of the opening in the radial direction. The engagement of the surfaces in combination with a force exerted by the screw on the bottom surface of the split-ring causes the internal diameter of the ring to decrease with increasing force from below. This insures the bone screw cannot back out of the opening.
In order to make the insertion of the bone screw easier, it is provided with an inclined surface complementary to an inclined surface on the internal bore of the split-ring, which inclined surfaces increase in diameter upon moving in a direction from the bottom surface of the plate towards the upper surface of the plate and radially outwardly of the opening central axis. Thus, when the screw head inclined surface engages the complementary inclined surface on the internal diameter of the split-ring, forces are generated which expand the split-ring into the groove. In order to increase the flexibility of the split-ring, at least one cutout and preferably three or more cutouts are spaced around the external diameter of the ring, resulting in a variable cross-section. This allows the ring to have more flexibility in expanding than if the external diameter of the ring were constant. In order to better prevent the egress of the bone screw from the plate, the surface of the split-ring facing towards the bottom of the plate is flat and extends generally perpendicularly to the central axis through each opening. The bone screw has a complimentary upwardly facing generally flat or slightly inclined surface.
The location of the groove in the plate is such that when the head of the screw fully engages the spherical seat in the plate, the upwardly facing surface is located below the bottom surface of the split-ring. In order to allow the bone screw to rotate from side to side once seated, an angular cutout of 0xc2x0 to 20xc2x0 can be provided at the bottom surface of the plate, thereby making the opening on the bottom surface oblong in at least one direction. This allows the longitudinal axis of the screw head and shank to be rotated between 0xc2x0 and 20xc2x0 with respect to the central axis of the opening.
The material for the split-ring must be flexible and be compatible with the body and it has been found that the titanium alloy disclosed in U.S. Pat. Nos. 4,857,269 and 4,952,236, which have modulus of elasticity not exceeding 100 GPa, is acceptable. Polymeric materials such as ultra-high molecular weight polyethylene are also acceptable.
The joining member or plate may be curved to match the anatomical curvatures. Thus, the implant curved to best suit the anatomy and natural curvature of the spinal column in the case of a spinal application. Of course, the plate may be used in fracture fixation, as a tibial baseplate, as a hip side plate or any application where bone plates and screws are used. For these uses, a larger screw than that described herein is necessary. The screw locking system can be scaled up from that described herein so that any size screw can be utilized in a smaller locking system.
Also envisaged is a method for implanting the implant involving accessing the spinal column via an anterior route, fitting the implant, preparing the anchorage, fitting the anchorage members, locking the implant and the head of the anchoring members with respect to the joining member, and closing up the access route.
These and other objects and advantages of the present invention will become apparent from the following description of the accompanying drawings. It is to be understood that the drawings are to be used for the purposes of illustration only and not as a definition of the invention.